The subject matter disclosed herein relates to refrigeration systems. More specifically, the subject matter disclosed herein relates to refrigeration of containers utilized to store and ship cargo.
A typical refrigerated cargo container or refrigerated truck trailer, such as those utilized to transport a cargo via sea, rail or road, is a container modified to include a refrigeration unit located at one end of the container. The refrigeration unit includes a compressor, condenser, expansion valve and evaporator serially connected by refrigerant lines in a closed refrigerant circuit in accord with known refrigerant vapor compression cycles. The evaporator is located in a compartment that requires cooling, such as a cargo compartment of a truck or trailer. The condenser and compressor are located outside of the compartment. Cargo compartment air is passed over the coils of the evaporator, vaporizing the refrigerant flowing through the evaporator coil, thus heat is absorbed from the air in the conditioned compartment to cool the conditioned compartment. The gaseous refrigerant is then flowed to the compressor for compression thereat. A power unit, including an engine, drives the compressor of the refrigeration unit, and is typically diesel powered, or in other applications natural gas powered. In many truck/trailer transport refrigeration systems, the compressor is driven by the engine shaft either through a belt drive or by a mechanical shaft-to-shaft link. In other systems, the engine drives a generator that generates electrical power, which in turn drives the compressor.
Many systems utilize natural gas fuels to power the engine. Natural gas fuels utilized include compressed natural gas (CNG), liquid natural gas (LNG) and propane. CNG, LNG, and propane, however, all require specialized storage and some operation on the fuel before its use at the engine. CNG is stored at ambient temperatures and a pressure of 3600 PSI. LNG is stored at ambient or close to ambient pressure but at a temperature of −273 F. Propane is stored at low pressure of around 300 PSI and ambient temperatures. Before these fuels can be used by the internal combustion engine they must be at ambient temperatures and ambient or close to ambient pressure. To accomplish this, CNG must be decompressed from 3600 PSI to about 10 PSI or less. This decompression process can cause temperature drop in the regulators and lines down to −60 F in some cases. For LNG the liquid must be heated from −273 F to ambient in order to regenerate the LNG into gaseous natural gas. This regeneration is typically achieved by use of an electric heater in the fuel system, or by utilizing waste heat from the engine to gasify the methane LNG. Propane is drawn as a liquid from the tank and heated to convert it into a gas.